KopyKitab Electric Motors

KopyKitab KopyKitab Electric Motors Applications and Control

M.V. DESHPANDE Formerly Professor & Head Department of Electrical Engineering L.D. College of Engineering, Ahmedabad Formerly Director of Technical Education Maharashtra State

KopyKitab

PHI Learning Private Limited New Delhi-110001 2010 ELECTRIC MOTORS: ApplicationsKopyKitab and Control M.V. Deshpande

© 2010 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher.

ISBN-978-81-203-3643-8

The export rights of this book are vested solely with the publisher.

Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Raj Press, New Delhi-110012. This work is respectfully dedicated to the memory of the author’s beloved mother Anandibai

KopyKitab KopyKitab Contents

Preface ...... xiii 1. Load Characteristics ...... 1–18 1.1 Introduction 1 1.2 Types of Loads 1 1.2.1 Load Torques Varying with Time 2 1.2.2 Load Torques Varying with Speed 2 1.3 Basic Equation of Motion for Drive System 2 1.4 Determination of Load Torque and Power Required by Load 4 1.4.1 Load Torque and Load Power 4 1.5 Determination of Moment of Inertia of Loads 5 1.5.1 Determination of Referred Moment of Inertia 6 1.5.2 Referring Forces and Masses having Translation Motion to a Rotating One 6 1.6 Speed–TorqueKopyKitab Curve of Load 7 1.7 Speed–Time or Acceleration–Time Curve 7 1.8 Environments of Motors 11 1.9 Load Characteristics for Some Typical Drives 12 1.10 Inertias of Load Connected to Motors 16 Exercises 17 2. Types of Motors and Their Characteristics ...... 19–55 2.1 Specification of Motors 19 2.1.1 Rating of Motors 19 2.1.2 Temperature Rise 20 vii viii Contents

2.2 Insulation Used for Motors 20 2.3 Classes of Duty 21 2.4 Temperature Rise: Heating and Cooling Curves 25 2.5 Rating of Motors 28 2.5.1 Continuous Duty and Constant Load 28 2.5.2 Continuous Duty and Variable Load 28 2.5.3 Short Time Rating of Motor 29 2.6 Types of Motors 31 2.7 Electrical Characteristics of DC Motors 32 2.8 Electrical Characteristics of AC Motors 37 2.8.1 Squirrel Cage Induction Motors 37 2.8.2 43 2.8.3 Double Squirrel Cage Induction Motors 45 2.8.4 Pole Changing Squirrel Cage Induction Motors 46 2.8.5 Three-phase Motors 48 2.8.6 Schrage Motor 48 2.8.7 49 2.9 Single-phase Motors 52 2.10 Types of Enclosures for Motors 53 Exercises 54 3. Starting and Braking of Motors ...... 56–89 3.1 Introduction: Motor Starting 56 3.2 DC Motor 56 3.2.1 DC Shunt Motor Starter 56 3.2.2 DC Series Motor Starter 59 3.3 Automatic Acceleration of Motor During Starting 62 3.4 Starting of Polyphase Induction Motor 62 3.4.1 The Methods Used for Starting Squirrel Cage Induction Motors 62 3.4.2 Wound Motor Starter 66 3.4.3 Resistance Steps in Slip Ring Motor Starter 67 3.5 Other Methods of Starting Induction Motor 68 3.5.1 InchingKopyKitab68 3.6 Starting of Synchronous Motor 69 3.7 Electric Braking Motor 69 3.7.1 Representation on Four Quadrant Diagram 70 3.8 Electric Braking of DC Motor 70 3.8.1 Braking of DC Shunt Motors 70 3.8.2 Braking of DC Series Motors 77 3.9 Electric Braking of Induction Motor 78 Exercises 87 Contents ix

4. Speed Control of Motors ...... 90–126 4.1 The Methods of Speed Control 90 4.2 Speed Control of DC Shunt Motor 92 4.2.1 Shunt Motor 92 4.2.2 Shunted Shunt Motor 92 4.3 Speed Control of DC Series Motor 95 4.4 Ward–Leonard Control 102 4.5 103 4.6 Magnetic Amplifiers 103 4.7 Dynamics of Induction Motor Starting 105 4.8 Load Equalisation 106 4.9 Speed Control of Squirrel Cage Induction Motors 110 4.9.1 Pole Changing 110 4.10 Speed Control of Slip Ring Type Induction Motor 111 4.10.1 Cascade Connections 111 4.10.2 Speed Control of Slip Ring Induction Motor by Resistance or Reactance in the Rotor Circuit 113 4.11 Slip Energy Recovery Systems 118 Exercises 123 5. Solid State Motor Control: DC Motor Speed Control ...... 127–153 5.1 Solid State Devices 127 5.1.1 Diode 127 5.1.2 Transistor 127 5.1.3 Thyristor 127 5.1.4 Different Modes of Thyristors 129 5.1.5 Methods of Triggering or Turning on Thyristors 130 5.1.6 Methods of Turning off Thyristors 130 5.1.7 Rating of Thyristors 131 5.2 Thyristor Circuits 133 5.2.1 Circuits to Turn on Thyristors 133 5.2.2 Circuits to Turn off Thyristors 135 5.2.3 ThyristorKopyKitab Protection 137 5.3 DC Motor Speed Control 137 5.4 Thyristor Conrolled Rectifier Converters 137 5.4.1 Uncontrolled Rectifiers 138 5.5 Thyristor Controlled Converters 139 5.5.1 DC Voltage Obtained by Different Converters 143 5.6 Voltage Relation in Thyristor Converters Circuits 143 5.6.1 Overlap 144 5.6.2 Three-phase, Phase Controlled Full Wave Bridge Rectifier 145 5.7 The Phase Controlled Inverter 145 5.8 Regeneration by Phase Control 147 x Contents

5.9 Integral Cycle Control 147 5.10 Chopper Control 148 5.10.1 Turn off with Chopper 148 5.10.2 Braking 150 5.10.3 Chopper Control Applications 151 5.11 Position Control by DC Motors 151 Exercises 152 6. Solid State Motor Control: Induction Motor Speed Control ...... 154–174 6.1 Classification of Inverters 154 6.1.1 Basic Inverter Configuration 154 6.2 Inverter Commutation Methods 155 6.2.1 Single-phase Bridge Inverter with Parallel Capacitor Commutation 155 6.2.2 Impulse Commutation Methods 156 6.2.3 McMurray-Bedford Method of Commutation 157 6.2.4 McMurray Method of Commutation 157 6.2.5 Input-circuit Commutation Method 158 6.3 Inverter Frequency Control 159 6.4 Voltage Control of Thyristor 160 6.4.1 Control of Alternating Voltage Output from the Inverter 160 6.4.2 Control of Direct Voltage Input to the Inverter 161 6.4.3 Voltage Control by Switching Techniques within the Inverter 163 6.5 Harmonic Elimination 164 6.6 Choice of Thyristor Three-phase Inverters 165 6.7 Inverter Control Circuitry 165 6.8 Phase Controlled 165 6.9 Frequency and Voltage Control of Cycloconverter 166 6.10 Comparison of the Cycloconverter and DC Link Converter 167 6.11 Induction Motor Speed Control 168 6.12 Regenerative Braking 169 6.13 The Controlled Slip Induction 170 6.14 Speed Control of Induction Motor: Control of Rotor Resistance 171 6.15 Inverters in InductionKopyKitab Motor–Rotor Circuit 171 Exercises 173 7. Selection of Motors for Industrial Drives ...... 175–201 7.1 Introduction 175 7.2 Electric Drivers for Textile Applications 175 7.2.1 General Guidelines for Selection of Motors 175 7.2.2 Standardisation of Motors 176 7.2.3 Selection of Motors for Various Textile Processes 176 7.3 Selection of Electric Motors for Machine Tool Applications 180 7.3.1 Standard Electrical Features and Mechanical Features 180 7.3.2 Frame and Mounting 181 Contents xi

7.3.3 High Slip Motors for Punch Presses, Shears, Hammers, etc. 182 7.3.4 Multispeed Motors 183 7.4 Electric Drive for Cranes 183 7.4.1 Classification of Cranes 184 7.5 Electric Motors in Explosive Atmosphere 188 7.6 Electric Motors for Compressor Drive 190 7.7 Electric Motors for Water Supply Systems 191 7.7.1 Parameters Considered for Selection of Pump Drive Motors 191 7.7.2 Performance Parameters of Motors 191 7.7.3 Selection of Control Gear for Pump Drives 193 7.8 Electric Motors for Power Station Auxiliaries 193 7.9 Electric Motors for Coal Mining 195 7.10 Electric Motors for Rolling Mills 196 7.11 Protection and Maintenance of Electric Motors 197 7.11.1 Protection of Motors for Specific Conditions 197 7.11.2 Maintenance of Motors 198 7.12 Motor Faults and Causes 199 Exercises 200 8. Economic Selection of Electric Motors ...... 202–210 8.1 Need for Conservation of Electrical Energy 202 8.2 Factors Affecting Efficiency of Induction Motors 203 8.3 Factors to be Considered for Efficiency of the System 204 8.4 High Efficiency Motors 204 8.5 Economics of Variable Speed Drives 205 8.6 Performance of Various Variable Speed Drives 207 8.7 Trends in the Development of Variable Speed Drives 208 8.8 Choice of Range of Variable Speed Drives 208 Exercises 210 Index...... KopyKitab ..211–213 KopyKitab Preface

The main objective of this book is to present the subject of ‘Applications and Control of Electric Motors’ to students studying for an electrical engineering degree as well as those undergoing diploma courses in electrical engineering. Starting with the motor–load system, the characteristics of motors and loads are explained with a view to selecting the correct type of motor for a particular application. The types of motors used, their characteristics, duty cycles, and specifications are discussed. The starting, braking and speed control of various types of motors are dealt with. The solid state speed control methods for DC and AC motors are discussed. The selection procedures of motors for various industrial drives are explained in detail with their applications. Finally, the economic viability of selection of motors is discussed. Throughout the book, the emphasis is laid on practical application of the theory to industrial motor drives and their problems. This book is an outcome of late Professor M.V. Deshpande’s lifetime teaching experience, who had felt the need of a book dealing with not only the conventional methods of control of electric motors but also the modern solid state methods that help conserve energy. The students of electricalKopyKitab engineering will find this book useful for their courses in ‘Electrical Power Utilization’ and ‘Electric Drives’. It will also be useful for AMIE examinations conducted by The Institution of Engineers (India). Electrical engineers in industry dealing with the application engineering problems in electric motors and electric drives will also find this book of help to them. The publisher is thankful to Dr. M.A. Joshi, daughter of late Professor M.V. Deshpande, for painstakingly proofreading the book.

Publisher

xiii C H A P T E R 1

Load Characteristics

1.1 INTRODUCTION While selecting suitable motors for industrial applications, a general understanding of the mechanical and electrical elements and the knowledge of load and motor characteristics is essential. There are two basic considerations in getting the motor-driven load started—the speed–torque characteristics of both the load and the motor and the load inertia characteristic, Wk2. Many loads need variable torque, while others require constant torque or constant power. Some loads can be started unloaded while others must be started on full load. Wk2 or the inertia problem will be lacking in some loads while some loads will have high Wk2 characteristics. The application engineer must thoroughly investigate all the requirements of the load and then select a motor with all the necessary characteristics to satisfy the requirements. To select the correct type of motor for a particular application, information on the following points must be available: (a) Inertia of the connected load referred to the motor shaft. (b) kW of load and KopyKitabthe length of time required for each period of running. (c) Time consumed in standing idle. (d) Number of stars, stops, reversals, etc.—diagram of duty cycle. (e) Type of braking used for stopping. (f) Relation between load torque and speed. (g) Other requirements such as high torque, space limitations, high ambient temperature, etc. It is, therefore, necessary (first) to study the types of loads and their characteristics.

1.2 TYPES OF LOADS The loads are of two types: those which provide active torques and those which provide passive torques. Active torques are due to gravitational force or deformation in elastic bodies. The active 1 Electric Motors: Applications And Control

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Publisher : PHI Learning ISBN : 9788120336438 Author : M. V. Deshpande

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